How to add materials_cycles_converter?
The Cycles Material converter addon is part of Blender Add-ons Contrib. The contrib addons are not part of the official blender releases, they are normally included with daily builds and some of the custom builds available at Graphicall.org. The contrib addons are considered still under development or not suitable to be accepted into the official addons for some reason. New addons will normally be found here for some time while being tested before they are included in the official addons.

If you want to install the cycles material converter by itself you can download the single script from blender's git repository by going to this page and clicking view raw file, then download file. Once it has been saved to your computer you can use the install from file... button in the preferences.

You can also download the entire contrib repository using git, by following the instructions on this page, or you can download a snapshot from here. Once this has downloaded (and extracted if you get a snapshot) you should place the contents into a folder called addons_contrib next to the addons folder installed with blender. While you can add them to the addons folder with the other addons, it can be easier to update them if you keep them separate as well as making it easy to identify which addons are official and which belong to contrib.

Also note that the contrib addons are listed under the testing category so you need to select that to see any contrib addons in the addons list.